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Caption

HIV retrovirus. Schematic illustration of the structure of a retrovirus, such as the HIV-1 virus that causes AIDS. At centre are RNA strands (rainbow colours) making up the viral genetic material. The RNA is enclosed in a core protein shell (yellow). The outer envelope (green) consists of a lipid bi-layer, with protein spikes (orange). In the HIV virus the spikes consist of a glycoprotein, gp120, and it is these that recognise CD4 markers on white blood T-cells, allowing the T-cells to become infected. The HIV virus kills T-cells after using them to reproduce its viral RNA, thus weakening the immune system of the human host.